1. Field of the Invention
The present invention relates to a radio communication system, centralized station, control station apparatus, radio base station and coordinated transmission method when transmitting a signal to a radio terminal by coordinating a plurality of radio base stations.
2. Description of the Related Art
Cellular-based mobile radio communication systems are required to decrease the frequency reuse as much as possible to improve area spectral efficiency. For example, a third-generation cellular scheme using a CDMA (Code Division Multiple Access) scheme realizes one-cell frequency reuse (i.e., the rate at which the same frequency can be used in the network=1) which uses a radio wave of the same frequency band between neighboring cells.
On the other hand, an OFDMA (Orthogonal Frequency Division Multiple Access) scheme is predominant for downlinks according to a next-generation cellular scheme. When one-cell frequency reuse is used in a cellular scheme using this OFDMA scheme, interference from a neighboring cell or neighboring sector is a dominant reason for performance degradation. To be more specific, as interference power from the neighboring cell or neighboring sector increases, an SINR (Signal-to-Interference and Noise power Ratio) decreases, and it is therefore difficult to obtain benefits thereof when performing MIMO (Multiple-Input Multiple-Output) transmission.
Therefore, when one-cell frequency reuse is used in a cellular scheme that performs MIMO transmission according to an OFDMA scheme, obtaining a notable benefit of increasing throughput by MIMO transmission requires interference from the neighboring cell or neighboring sector to be avoided.
Thus, as an interference avoidance technique in the above described case, coordinated transmission whereby signals are simultaneously transmitted to one or more radio terminals by coordinating a plurality of radio base stations is becoming a focus of attention. When a precoding method for multiuser MIMO transmission is used, it is possible to orthogonalize the space in a coordinated cluster formed of a set of cells or sectors of a plurality of coordinated radio base stations and thereby avoid interference in the coordinated cluster (that is, interference between radio base stations that perform coordinated transmission) (e.g., Non-Patent Document 1: A. Benjebbour, M. Shirakabe, Y. Ohwatari, J. Hagiwara, and T. Ohya, “Evaluation of user throughput for MU-MIMO coordinated wireless networks,” IEEE PIMRC 2008, pp. 1-5, September 2008. Non-Patent Document 2: CMCC, “Downlink CoMP-MU-MIMO transmission schemes,” 3GPP RAN1 #56, R1-090922, February 2009).
To be more specific, in the coordinated transmission described in Non-Patent Document 1, a plurality of neighboring radio base stations are connected to a centralized station and the centralized station coordinates the plurality of radio base stations. The coordinated cluster is formed of a set of cells of a plurality of coordinated radio base stations. In the coordinated cluster, the plurality of radio base stations perform coordinated transmission using multiuser MIMO transmission. When a block diagonalization Zero-forcing (BD-ZF) method is used as a precoding method of multiuser MIMO transmission, it is possible to orthogonalize the space in a coordinated cluster CL, and thereby avoid interference in the coordinated cluster CL and improve user throughput.
Furthermore, in the coordinated transmission described in Non-Patent Document 2, as shown in FIG. 1, three neighboring radio base stations (1-1, 1-2, 1-7), (1-1, 1-3, 1-4), (1-1, 1-5, 1-6) are connected to a centralized station 2 and in communication with a radio termina, the centralized station 2 coordinates three radio base stations which control an area in which the radio terminal is present. A coordinated cluster CL is formed of three neighboring sectors of three coordinated radio base stations 1. Furthermore, the three neighboring sectors constituting the coordinated cluster CL are configured so that beams from sector antennas of each radio base station 1 face each other. In the coordinated cluster CL, coordinated transmission is performed by the three radio base stations 1 through multiuser MIMO transmission. Use of a precoding method for multiuser MIMO transmission allows the space in the coordinated cluster CL to be orthogonalized, can thereby avoid interference in the coordinated cluster CL and improve user throughput and cell throughput.
However, the aforementioned interference avoidance technique can avoid interference in the coordinated cluster, but has a problem that the technique is unable to avoid interference between coordinated clusters.